An imaging system is, for example, a computed tomography system. A computed tomography system comprises a radiation source emitting radiation and a detection unit for generating projection data which depend on the intensity of the radiation after having traversed an object to be imaged, wherein the radiation source and the detection unit are rotated around the object, in order to acquire the projection data in different projection directions. The computed tomography system further comprises a reconstruction unit for reconstructing an image of the object from the acquired projection data.
The radiation source comprises a target material with a primary focal spot region, towards which electrons are accelerated by an acceleration field. The accelerated electrons meet the target material at the primary focal spot region and generate thereby primary radiation, which emanates from the primary focal spot region, and secondary electrons as a by-product. Some of these secondary electrons escape from the target material and are flung back towards the target material by the acceleration field. The secondary electrons meet the target material and generate thereby secondary radiation emanating from secondary focal spots regions. This secondary radiation adversely affects the acquired projection data and reduces therefore the quality of the acquired projection data, thereby also reducing the quality of the image of the object, which is reconstructed from the acquired projection data.